If this is your first visit, be sure to
check out the FAQ by clicking the
link above. You may have to register
before you can post: click the register link above to proceed. To start viewing messages,
select the forum that you want to visit from the selection below.

rotary table project in pvc

Here's the start of some pics of this project. Going down the left side, the first two pix are of the mostly completed rotary table. The second pic shows the only controls, the one on the left being the release to allow the table to turn. Below that is the bottom of the rotating part. On the right, the first pic shows the bearing that the table turns on, and the two end pieces that serve to keep the table down on the base. There's a slice taken out of the inside of these- that allows the center bolt to bend the plastic down a bit, and I'm using this to bite down on the table so it can't rotate. If I loosen both center bolts, and push down on the control on the left, the table is disengaged and free to be rotated by hand.
The other control, which is shown better in the next pic down, is the micro adjustment of the tables position. It's shown in the 'zero' condition, but can be adjusted for one full degree of change. Each marking represists 1/12 of a degree, or 5 minutes of arc. The last pic here shows the short threaded rod section which indexes to the bottom of the table. Near the center of that is a brass cam that when turned pushes that piece down against a flat spring, disengaging it from the table. On the far right of that short arc is a return spring, and on the far left you can just see the end of a brass rod pushing against that end of the arc. The adjustment knob turns a bolt against that rod, thus the arc piece has a range of motion. Going back to the top right pic, you see that the blue piece goes under the bearing. It actually pivots on its own boss under the bearing. There's a bolt protruding from the center of the bearing- that's used simply to press the table off that bearing and is removed before it's assembled. It plays no other role.

The top pic is a close-up of the threaded rod circle which is epoxied into the slot on the back of the table. As you can see in the next pic, the arc is plenty long enough to bridge this gap, thus there are no 'bad spots' which can't be indexed. While fitting and gluing the circle of all-thread, I pressed the short arc against the circle and spanning the gap to precisely space the gap such that there is no spacing inconsistency between threads.
The last two pix show what happens when I turn the knob- in the first the knob is up, the arc is up, and the brass cam is up. In this positon, the arc locks into the threads in the table, and the table can only be turned via the adjusting knob. In the second, I'm pushing the knob down. Note how the arc is displaced lower into the base- the blue plastic that the arc is epoxied into is thinned in its center to allow for this flex.
This is also the best pic I took of the brass 'cam'- actually it would be better described as a crank pin.

What's left to do is mark out the circumference of the table similarly to a protractor, and mark out maybe a couple of other useful divisions that aren't easily read on a degree scale. This will make it quicker and easier to index without having to count each step. Where I might need to divide a circle into say 48 parts, I'll need to use the micro adjustment to get half degrees (7.5 degrees per step). If I use each marking on the adjustment knob, a circle will divide into 4320 parts, and if I interpolate between markings I'm up to 8600 plus divisions capable.

The table has a precise hole in the center- this hole was used to pivot the piece on to mill the hole for the bearing, so it's precisely concentric. There are a number of ways I can see to mount a workpiece so it's concentric with this hole, and I'm sure there will end up being several tapped holes in the table to accomodate the job at hand. One of the first jobs will be marking out an angle finder in fractions of a degree in a range of about 3 degrees surrounding 90, 45, and 22.5 degrees. I need this at work. I will probably mount a sacrificial plate on top of the table so the pvc doesn't get marked up.

Oh, and why pvc? Cuz I have it, cuz it has proven to be fairly stable dimensionally, because I don't intend to use this where machining forces are acting against the table rotation (downward forces would be fine), and marking out won't take much force so it's plenty strong enough. It doesn't gall like aluminum, and it machines easily and beburs easily. Plus- I actually like the Fisher Price look

I've read where a child could eat a half pound of pvc with no ill effects, but I'm not going to experiment with that. To some extent I have to believe that simply because it's so non-reactive to many liquids. At any rate, this little project becomes a highly accurate cake decorating machine

I'm going back to see if I can still edit the first two posts. If I have been sucessful, many answers will be found there.

Thanks for the comments on my workmanship. It is nice to work with this material- one of the reasons is that with a sharp cutting tool, there's essentially no give in it. You can measure a diameter and if it's say 1 thou large, you can dial in 1/2 thou and clean that off without finding that you've gone too deep. You can certainly have that problem also, especially if the tool is dull or you use too much back rake. When I milled around the circumference by spinning the disc on a pivot point, I was approaching the precise diameter in about 1/2 thou steps, and the end mill took it off precisely each time. It's not a hard cut either, so deflection of the cutter or workpiece is almost not there at all. Just to remind though, pvc dulls the tool fairly quickly, especially with high cutting speeds. With the end mill I have no problem because the speeds are relatively slow, though I could belt it up to the max and it would cut fine. I would lose the endmill sooner though. On the lathe I have to sharpen frequently because the sfms are usually much higher. I haven't yet found a point where the speed is too fast for cutting. (except for this dulling of the tool).

The diameter of this table is 6 5/8 inches. This accomodates the length required for the circle of threaded rod, which needs to be 18 inches long for the 360 threads per circle. The threads here are only used to index with, and nothing drives against it to rotate the table. That's strictly a hand powered thing- loosen the two screws, press the disengagement lever and turn the table by hand to the required point, then release the lever and feel the threads lock in. Tighten the screws and go.

One interesting thing I discovered, not unexpectedly, is that the short arc section of threads engages the circle properly only if it's radius from center is exactly equal to the radius of the circle. In other words, if you try to move it towards or away from center, it will climb out of the threads. I used this property to align the short arc while epoxying it into place. It aligned itself perfectly, and in use it locks in positively with no mushiness at all, and happily for me, it seems to do so anywhere around the circumference. That tells me the threads are quite accurately spaced along the length of the threaded rod circle. In turn that suggests to me that the accuracy of the degree points around the circle will be quite high. I have no problem seeing that my 5 minutes of arc divisions will be accurate as well. I can't forsee needing 4320 divisions around anything, but to have this accuracy means I can get divisions that aren't exact submultiples of 360. If I need 100 divisions, for example, that's 3.6 degrees per- if I change the adjustment wheel to show 10 divisions rather than 12, which I intend to do, I can dial that in easily, and I know it will result in equal divisions. I only went with 12 divisions around my dial because that was easy with the dividing disc on my lathe. I can't do 100 divisions with that. Not yet anyway.

I plan to mark out the degrees on the top edge of the table, and other divisions around the side of the table. I'll scratch out these marks, spray paint around them, then scrape the surface again. That will clean up, leaving the paint in only the scratches. Should be highly visible. The tedious part will be rotating the table by degree to start with to get these marks in place. I'll have to feel the clicks of the indexing system and count off, probably in groups of five to start. Once I have that I won't ever have to count the degrees again. If I use a sacrificial plate on top of this table that is larger than the table, it will hide the degree markings, but I'll have the side markings to use still visible.

I'll probably do a couple of mods to it. I don't like the look of the knobs (one I'm re-doing anyway) so I'll change that. No big deal.

David, I have a couple applications where I need to make up some degree measuring equipment. These are angle finders, basically, but they need to read in fractions of a degree. In our countertop cutting job, it's often needed to read a fraction of a degree so that a 90 degree corner with long countertops will fit without requiring re-cutting of the angles. Corners are seldom square, and if you have to scribe too much off the backsplash it doesn't look good. An error of 1 degree in a corner shows up as a gap of about an inch over a five foot length of countertop. Our machine will cut to within a tenth of a degree easily, so we want to be able to measure any angle in a house with that same degree of accuracy.

That's one thing. I also need to cut some odd angles on the tablesaw at times. If it's 30 or 45 degrees, I'd just use a plastic triangle to help me orient the piece to be cut, but I need 20 degrees sometimes, 43 degrees at times- so I'm going to use this table to help me position pieces on the mill so I can cut these gauges to the exact angle I need. When fitting more than a few pieces together, it doesn't take many compounded errors to throw things off. I'll use these custom triangles I make to aid in the positioning of the material I'm cutting.

I'm looking at making some very small plastic gears of some exact tooth count for another future project. I'll be able to do that on this table as well.

Sometimes I feel the need to be creative, and I started this project largely because of that, so I suppose one thing I'm using it for is as an outlet for my creative energies. If I could retire right now I'd probably go nuts with the projects I've wanted to do and either never had the time, the knowledge, the capabilities, etc. I'm in a position now where I have much of the machinery I would need to do most any operation I would want to, so the more gear I have to be able to do the various things, the more reasonable it becomes to do some of these projects. Some are pretty offbeat, but that's my character.